1. Technical Field
The invention generally relates to hydraulically driven pumps, and more specifically relates to diaphragms for hydraulically driven pumps.
2. Related Art
The known rotary-operated, oil-backed/driven diaphragm pump is a high-pressure pump inherently capable of pumping many difficult fluids because in the process fluid, it has no sliding pistons or seals to abrade. The diaphragm isolates the pump completely from the surrounding environment (the process fluid), thereby protecting the pump from contamination.
In general, a diaphragm pump 20 is shown in FIGS. 9 and 10. Pump 20 has a drive shaft 22 rigidly held in the pump housing 24 by a large tapered roller bearing 26 at the rear of the shaft and a small bearing (not shown) at the front of the shaft. Sandwiched between another pair of large bearings (not shown) is a fixed-angle cam or wobble plate 28. As the drive shaft turns, the wobble plate moves, oscillating forward and back converting axial motion into linear motion. The three piston assemblies 30 (only one piston assembly is shown) are alternately displaced by the wobble plate 28. As shown later, each piston is in an enclosure including a cylinder such that the enclosure is filled with oil. A ball check valve 32 in the bottom of the piston/cylinder assembly 30 functions to allow oil from a reservoir 27 (wobble plate 28 is in the reservoir) to fill the enclosure on the suction stroke. During the output or pumping stroke, the held oil in the enclosure pressurizes the back side of diaphragm 34 and as the wobble plate moves, causes the diaphragm to flex forward to provide the pumping action. Ideally, the pump hydraulically balances the pressure across the diaphragm over the complete design pressure range. As discussed later, in actual practice this is not the case for all situations for known pumps. In any case, each diaphragm has its own pumping chamber that contains an inlet and an outlet check valve assembly 36, 37. As the diaphragm retracts, process fluid enters the pump through a common inlet and passes through one of the inlet check valves. On the output or pumping stroke, the diaphragm forces the process fluid out the discharge check valve and through the manifold common outlet. The diaphragms, equally spaced 120° from one another, operate sequentially to provide constant, virtually pulse-free flow of process fluid.
The diaphragm 34 is held between two portions 38, 40 of housing 24. Diaphragm 34 separates the pump side from the oil-filled, hydraulic drive side of the pump. On the drive side, a drive piston assembly 30 including a diaphragm plunger 42 is contained within the oil filled enclosure which functions as a transfer chamber 44. A pair of check valves 32 in piston 46 separate transfer chamber 44 from the oil reservoir (not shown). Wobble plate 28 (not shown in FIG. 2) contacts pad 48 to drive piston 46. Arrow 49 indicates the general direction of movement of the cam or wobble plate. When the piston and diaphragm have finished the forward or pumping stroke, the end 50 of piston 46 is at top dead center (TDC). When the piston and diaphragm have retracted in the suction stroke, the end 50 of piston 46 is at bottom dead center (BDC).
FIGS. 11(a)–(f) illustrate operation of the conventional pump 20 under normal, standard operating conditions using a conventional bias spring 96. Typical pressures are shown. Typical vector directions for the cam or wobble plate (not shown in FIGS. 11(a)–(f)) are shown. Suction is less than 14.7 psia. Output pressure is greater than 14.7 psia. The pressure differential across diaphragm 34 is set at about 3 psia.
With reference to FIG. 11(a), the suction stroke begins at the end of the pumping stroke. For the conditions assumed, pressure in the pumping chamber immediately drops from what it was at high pressure, for example, 120 psia to 10 psia. Pressure in the hydraulic transfer chamber is 13 psia, which is less than the 14.7 psia in the reservoir. The piston 30 is at top dead center and begins moving toward bottom dead center. Bias spring 96 momentarily moves plunger 42, and particularly valve spool 84, to the right to open port 98. Because pressure in the transfer chamber is less than the pressure in the reservoir, check valve 46 opens and oil flows from the reservoir to the transfer chamber to appropriately fill it with oil that had been lost during the pumping stroke previous. That is, under the pressure of the pumping stroke oil flows through somewhat loose tolerances of the parts of the piston so that the some oil flows from the transfer chamber back to the reservoir. Thus oil needs to be refilled in the transfer chamber during the suction stroke so that there is enough oil to efficiently provide pressure during the next pumping stroke.
FIG. 11(b) shows the configuration at mid-stroke. The slight suction in the pumping chamber (shown to be 10 psia), holds diaphragm 34 and spool 84 to the left while piston 30 moves to the right, thereby shutting off port 98. Since pressures are nearly equal and diaphragm 34 moves right with piston 30, the pumping chamber fills with process fluid.
As shown in FIG. 11(c), process fluid continues to fill as diaphragm 34 moves right. Valve port 98 remains shut. Very little leakage of oil occurs from the reservoir (not shown) to transfer chamber 44, since pressures are nearly equal. Thus, both sides of the diaphragm fill properly.
When piston 30 reaches bottom dead center, the suction stroke is completed and the output or pumping stroke begins. Pressure in the transfer chamber immediately increases, for example, from 13 psia to 123 psia. Likewise, pressure in the pumping chamber immediately increases, for example, from 10 psia to 120 psia. The wobble plate begins moving piston 30 to the left, which causes the build-up of pressure. Check valves 32 close. Diaphragm 34 moves in volume tandem with the oil and process fluid left with the piston to push (pump) process fluid out.
At mid-stroke as shown in FIG. 11(e), there is continued output. Some oil leakage past the tolerances between piston and cylinder may move valve spool 84 of diaphragm plunger 42 to the right to open valve port 98. Check valves 32, however, are closed, thereby locking the oil in transfer chamber 44, except for leakage.
The output stroke finishes with the configuration shown in FIG. 11(f). The filled transfer chamber 44 pushes diaphragm 32 to the left dispensing process fluid as it moves. Normal operation as shown in FIGS. 11(a)–(f) causes little stress on diaphragm 32.
Piston 46 reciprocates in cylinder 47. Piston 46 has a sleeve section 52 that forms the outer wall of the piston. Sleeve section 52 includes a sleeve 54 and an end portion 56 at the end having pad 48 that is contact with the wobble plate. Within sleeve 54 is contained a base section 58. Base section 58 includes a first base 60 that is in contact with end portion 56 and includes seal elements 62 for sealing between first base 60 and sleeve 54. Base section 58 also includes second base 64 at the end opposite of first base 60. Connecting wall 66 connects first and second bases 60 and 64. Piston return spring 68 is a coil spring that extends between first base 60 and diaphragm stop 70, which is a part of the pump housing 24. Valve housing 72 is contained within base section 58 and extends between second base 64 and end portion 56. Seals 74 provide a seal mechanism between valve housing 72 and connecting wall 66 near second base 64.
The end 76 opposite end portion 56 of sleeve portion 52 is open. Likewise, the end 78 of valve housing 72 is open. Second base 64 has an opening 80 for receiving the stem 82 of plunger 42.
Diaphragm plunger 42 has the valve spool 84 fitted within valve housing 72 with the stem 82 extending from the valve spool 84 through opening 80 to head 86 on the transfer chamber side of diaphragm 34. Base plate 88 is on the pumping chamber side of diaphragm 34 and clamps the diaphragm to head 86 using a screw 90 which threads into the hollow portion 92 of plunger 42. Hollow portion 92 extends axially from one end of plunger 42 to the other end. Screw 90 is threaded into the diaphragm end. The piston end of hollow portion 92 is open. A plurality of radially directed openings 94 are provided in stem 82. A bias spring 96 is a coil spring and extends between second base 64 and valve spool 84. A valve port 98 is provided in the wall of valve housing 72. A groove 100 extends in connecting wall 66 from the furthest travel of valve port 100 to end portion 56. A check valve 102 is formed in end portion 56 in a passage 104, which is fluid communication with the reservoir (not shown). Thus, there is fluid communication from the reservoir (not shown) through passage 104 and check valve 102 via groove 100 to valve port 98. When the valve is open, there is further communication through the space in which coil spring 96 is located and then through one of the plurality of radial openings 94 and through the axial hollow portion 92 of plunger 84. There is further fluid communication from the hollow portion 92 through the other radially directed openings 94 to various portions of transfer chamber 44. The hollow passage 92, along with the radially directed openings 94 provide fluid communication from the portion of transfer chamber 44 near diaphragm 34 to the portion of transfer chamber 44 within the valve housing 72 of piston 30. The transfer chamber also includes the space occupied by piston return spring 68.
On the pump side of diaphragm 34, there is an inlet check valve assembly 36 which opens during the suction stroke when a vacuum is created in pumping chamber 106. There is also a check valve 37 that opens during the pumping or output stroke when pressure is created in pumping chamber 106.
A problem with conventional diaphragm pumps, however, is an unexpected diaphragm rupture under certain operating conditions. The diaphragm can fail much sooner than normal, or more frequently, may fail sooner than other pump components. A failure contaminates the process lines with drive oil. The operating condition that most often causes failure is a high vacuum inlet with a corresponding low outlet pressure. This is an expected occurrence in a typical pumping system when the inlet filter begins to plug. In that case, the plugging requires high vacuum to now pull process fluid through the filter. At the same time, the lowering of process fluid volume pumped drops the outlet pressure. This creates a situation where a high suction on the pumping side lowers the pressure during the suction stroke on the transfer chamber side so that the transfer chamber essentially “asks for more fill fluid” and, consequently, in-flowing oil overfills the transfer chamber and does so without a corresponding high pressure to push oil out during the pumping or output stroke to counter-balance. The overfill of oil “balloons” the diaphragm into the fluid valve port until the diaphragm tears. Additionally, with a high-speed, reversing, vacuum/pressure pump such as this apparatus, the high-speed valve closings create tremendous pressure spikes, called Joukobski shocks. The spikes can consist of fluid pressure or acoustical waves and harmonics of both. These pressure spikes can “call for” oil fluid flow into the drive piston when that should not be happening. Again, this can cause overfill and lead to the diaphragm failure.
Because of the possibility of diaphragm rupture, hydraulically driven pumps are typically unsuitable for many applications that cannot tolerate this type of contamination, such as, for example, spray paint lines. Except for the possibility of contamination due to a ruptured diaphragm, hydraulically driven pumps would be a preferred choice of pumps for many applications. Therefore, there is a need to be able to detect when a diaphragm fails and use that information to shut down the pump.
Various methods for detecting diaphragm rupture have been employed in the past. One example diaphragm rupture detection system includes the use of two diaphragms that have a vacuum between them, as disclosed in U.S. Pat. No. 5,667,368. This method requires the use of complicated hardware for maintaining a vacuum and detecting the loss of vacuum.
Another diaphragm rupture detection method uses two diaphragms that are separated by a liquid. This method, as disclosed in U.S. Pat. No. 4,971,523, includes detecting a change in the electrical conductivity of the liquid. This method further requires that the properties of the liquid are different from the pumped fluid so that a change can be detected in the event of a rupture. Thus, a requirement of this method is that intermediate fluid is different than the pumped fluid and the pump oil, which intermediate fluid may have to be changed depending on the conductivity of the pumped fluid.
Another method of diaphragm rupture detection is disclosed in U.S. Pat. No. 4,781,535. This method uses a continuous metal trace formed on the surface of a diaphragm. The trace is monitored for continuity as well as ground fault to the pumped liquid. The ground fault detection is to guard against breakage being masked by the conductivity of the fluid, which would otherwise bridge the conductive trace. Since the trace is required to be metallic, the trace will break if it is exposed to any significant strain. This system enables the use of a metal trace by minimizing the strain through the use of complex trace shapes. U.S. Pat. No. 4,781,535 illustrates several example low deflection, large diameter diaphragms that have relatively low strain.